SysV Init (Cont.)
A "run-level" is simply a state to put the machine in, and defines a set of services that should be running. For example, one run-level may be used to place the system in 'single-user' mode. Another to have the X server running, with 'xdm' used as the login program, and so on.
Since "init" uses shell scripts to launch the processes in a sequential manner, if one script gets stuck, no other script afterwards will be launched. A good example is launching of sendmail on not-connected systems. it often tries to make DNS lookup of the local hostname before moving to the background, thus blocking the startup process for 30 or more seconds.